Cigarette smoking continues to be the nation's most important public health problem, accounting for 350,000 deaths each year. Most smokers realize the dangers of smoking, and nearly 22 million try to quit each year. Unfortunately, 30% of those who try to quit relapse in the first week after cessation, 45% relapse by 2 weeks after cessation, 60% by 1 month, and 80% by 1 year after cessation. Very little is known at present concerning factors which might precipitate relapse, but variables from both the biological and behavioral domains are thought to be involved. Given the huge relapse rates in the early days and weeks after cessation, it seems essential that prospective studies be undertaken to examine this early period intensively to determine the biological and behavioral precipitants of relapse. If relapse could be prevented, we would be well on the way towards eradicating the smoking problem in this country. We have formed an interdisciplinary team of psychologists, endocrinologists, and biostatisticians to provide one of the first prospective studies involving an intensive examination of the early period after cessation, and collection of both biological and behavioral data. Biological variables are especially important to collect because their relationships to relapse have not been studied previously. It is generally agreed that cessation of smoking has definite biological effects, but the hypothesis that relapsers suffer more severe biological effects than successful quitters has never been tested. This hypothesis is central to our study, and its establishment or refutation has profound implications for the types of treatments devised to help smokers break the tobacco habit. Our study will be entirely prospective, and 300 smokers will be followed for 1 year after a quit attempt, with special emphasis placed on data collection in the early days and weeks after cessation. Results should provide valuable prospective information on biological and behavioral factors in smoking relapse. Particularly exciting is the possibility of discovering differential patterns of biological effects for relapsers and quitters. Such a finding could suggest treatment modalities helpful for sustaining cessation in the very difficult early period after quitting when most relapses occur.